A diminutive new basilosaurid whale reveals the trajectory of the cetacean life histories during the Eocene

Soon after whales originated from small terrestrial artiodactyl ancestors, basal stem forms (archaeocetes) came to inhabit more specialized aquatic ecologies and underwent a tremendous adaptive radiation that culminated in the adoption of a fully aquatic lifestyle. This adaptive strategy is first documented by the geographically widespread extinct family Basilosauridae. Here we report a new basilosaurid genus and species, Tutcetus rayanensis, from the middle Eocene of Fayum, Egypt. This new whale is not only the smallest known basilosaurid, but it is also one of the oldest records of this family from Africa. Tutcetus allows us to further test hypotheses regarding basilosaurids’ early success in the aquatic ecosystem, which lasted into the latest Eocene, and their ability to outcompete amphibious stem whales and opportunistically adapt to new niches after they completely severed their ties to the land. Tutcetus also significantly expands the size range of the basilosaurids and reveals new details about their life histories, phylogeny, and paleobiogeography.

) The data matrix from Martínez-Cáceres et al. 20 was simply expanded by implanting Tutcetus rayanensis (MUVP 501). As a result, 32 taxa with 101 characters (95 ordered) were incorporated into Supplementary Data (2). This matrix seeks to establish the relationships of MUVP 501 among the Basilosauridae and focuses on the relationships of the more derived archaeocetes in general and on the Archaeoceti-Neoceti transition.
We prefer Bayesian methods over parsimony to estimate phylogenetic topology because the former can combine various types of data (such as temporal and stratigraphic information, both parsimony informative and uninformative characters, rates of morphological evolution, and an underlying model of evolution) into a single analysis. Furthermore, under various circumstances, Bayesian methods can outperform parsimony [21][22] . The tip-dating trees obtained reveal topological variations between the two matrices utilized in this investigation, which are probably caused by assumptions about the evolution of morphological characters, the uncertainty brought on by missing data, and a lack of taxon overlap.

A. Extended Description
The cranium was found upside down in the field. Premaxilla: A 10 cm part of the premaxilla is detached from the rest of the cranium. Between the alveoli of the incisors, the premaxilla has deep laterally opened embrasure pits that receive the crowns of the lower incisors. Nasal: The roof of the narial cavity is represented by the nasals, which are nearly complete in MUVP 501. While the nasals articulate with the maxilla and frontal laterally, they meet medially along the midline. It is unclear where the anterior nasals end in relation to the tooth row because the ventral surface of the rostrum is severely damaged. Vomer: The anterior 30 mm of the vomer can be seen from the ventral surface with a height of around 23 mm. When the vomeral wings meet in the middle, a sharp ridge arises, which serves as a septum, separating the two narial passages. This division makes the vomer triangular in crosssection. Presphenoid: The presphenoid portion of MUVP 501's cranium is a robust midline element that is 87 mm in length. It had an anterior breadth of about 18 mm and widens posteriorly to a width of about 40 mm, where it had cartilaginous contact with the sphenoid. A thin layer of frontal bone covers most of the lateral sides of the presphenoid. Basisphenoid: Anteriorly, at its connection with the presphenoid, the basisphenoid has a pentagonal cross-section, measuring approximately 40 mm wide. It widens posteriorly as it approaches the cerebral cavity, where it was supposed to touch the missing basioccipital and make the floor of the braincase. Alisphenoid: In the lateral view of the skull of MUVP 501, the alisphenoid can be seen as a part of the anterior medial wall of the temporal fossa. The sphenoparietal suture, which rises anteriorly and dorsally, connects the alisphenoid to the frontal anteriorly, the parietal, and the squamosal posteriorly. Frontal: The frontal articulates posteriorly with the parietal and anteriorly with the nasal and the maxilla. The frontal (supraorbital) shield is formed by the frontal expanding laterally. The frontal shield has a frontal foramen on its posteroventral border. The medial borders of the frontals divide them along the midline, while the posterior edge of the maxillae bordered them anteriorly. Parietal: The dorsolateral portion of the cranium is formed by the parietal. The parietooccipital suture runs posteroventrally until it reaches the squamosal. Left and right parietals articulate along the midline to form a strong and prominent sagittal crest that runs from the apex of the nuchal crest posteriorly and continues to the frontoparietal suture anteriorly. Occipital: The exoccipital portion of the occiput is missing on the right side.

Squamosal:
The posterolateral wall of the cranium is formed by the squamosal. It contacts anterodorsally with the parietal, anteriorly with the alisphenoid, posteriorly with the exoccipital, and medially with the anterior processes of the periotic. The squamosal forms the lateral wall of the braincase and the glenoid and squamosal fossae. Much of the medial surface is in contact with the anterior process of the periotic. Tympanic bulla: A piece of the involucrum of the left tympanic bulla is still embedded in the basicranium, while a nearly complete isolated right tympanic bulla is preserved, and therefore the description is based on the right bulla. Tutcetus has a slightly convex medial edge on its tympanic bulla, in contrast to protocetids, which have a concave medial margin 23-25 . The lateral posterior eminence is longer and more pronounced than the medial posterior eminence. The medial eminence is the posterior part of the involucrum of the bulla. The tympanic cavity is dorsally open, narrowest posteriorly, and widens toward the anterior border of the bulla. The involucrum is convex on the medial, posterior, and dorsal sides. The involucrum is divided into two halves, one more bulbous on the posterior side and the other relatively low on the anterior side. The convexity of the posterior half is higher than the anterior part.

Mandible:
The mandibular condyle of the left mandible is not preserved. The dorsal surface of the dentary continues to extend upward toward the apex of the coronoid process. The edge of the coronoid process turns ventrally again and curves just slightly anteriorly, leaving the coronoid process projecting both dorsally and posteriorly. The posterior edge of the mandible continues ventrally, forming the mandibular notch, between the coronoid process and the mandibular condyle, which is just ventral to the lower border of the coronoid process. The articular surface of the right condyle is generally subtriangular in dorsal view and its surface is broadly convex. The entire articular surface is angled at about 45 • to the horizontal ramus, sloping down and back. The condyloid crest arises from the mandibular condyle and extends anteriorly along the lateral face of the dentary to around P4.
Dentition dC1: While the left deciduous lower canine (dC1) is exposed, the right one is unexposed but can be distinguished using the CT scan. Hence, the left lower deciduous canine is used as the basis for the description. There are no well-developed ridges or cingula on the crown.

Upper posterior premolars (P 3-4 ):
Both the upper posterior premolars (P 3-4 ) have a distolingual expansion on the distal root and crown, a remnant of the protocone that is present in nonbasilosaurid archaeocetes.

Upper molars (M 1-2 ):
The upper molars are much smaller than the preserved upper premolars.

Lower incisors (I2-3):
The left and right third incisors both have an angle of about 150 • between the longitudinal axis of the root and the crown, while the left second incisor has an angle of 140 • . Lower second premolar (P2): The crown is straight and triangular. There is a 16 mm diastema separating P2 from the posterior lower premolars. Lower posterior premolars (P3-4): The mesial side of the left third lower premolar is severely damaged. The enamel is smooth.

Hyoid apparatus
Stylohyal: MUVP 501 has a 105-mm long and 7.5-mm-wide stylohyal. Thyrohyal: The thyrohyal is estimated to be 62 mm long, shorter than the stylohyal. The proximal end is much wider (15 mm) than the distal, nearly cylindrical end. The thyrohyal narrows (8 mm) posterior to this wide end, forming the almost cylindrical and almost straight thyrohyal body.

Atlas (C1):
The atlas, the first cervical vertebra (C1), does not have a vertebral body and is similar to those of other basilosaurids 1 . While the ventral arch presents two highly concave articular foveae cranially that articulate with the occipital condyles of the skull, it has a broad convex articular surface caudally that receives the odontoid process of the axis. The ventral arch extends laterally and caudally, forming the transverse process. The articular surfaces are connected ventrally and separated dorsally by a wide supracondylar notch.

B. Phylogenetic Analyses
Bayesian tip-dating analysis of Supplementary Data (1): Included in the main manuscript (Phylogenetic relationships). Our phylogenetic analysis of Supplementary Data (1) demonstrates a notable affinity between pachycetines and Neoceti. However, the phylogenetic position of pachycetines remains somewhat uncertain due to the limited availability of well-preserved specimens. Pachycetines exhibit some basal traits among basilosaurids, such as relatively thicker posterior mandible walls and primitive auditory region anatomy. Nevertheless, their affinity with Neoceti may arise from specialized adaptations for utilizing low-frequency sounds, similar to those found in Miocene and extant baleen whales 26 . Our results highlight the shared anatomical features between pachycetines and both early and modern Neoceti 27 . For instance, some pachycetines (e.g., GMTSNUK 2638) (Supplementary Table 1) display an anteriorly inclined supraoccipital shield, an autapomorphy of Neoceti. Furthermore, the maxillae diverge from the typical basilosaurid morphology 1 , bearing resemblance to the earliest mysticete, Mystacodon selenensis [28][29] . The tooth row also terminates more anteriorly than in basilosaurids such as Pontogeneus peruvianus 23 and Dorudon atrox 1 , with the alveolus for the rearmost tooth situated directly ventral to the lacrimal canal-similar to Mystacodon selenensis 28 . These shared features offer persuasive evidence for an evolutionary relationship between pachycetines and Neoceti. However, additional, more comprehensive specimens are required to ascertain the relationship between these groups.

Bayesian tip-dating analysis of Supplementary Data (2):
The second, and taxonomically more restricted, BTD analysis recovered Georgiacetus as the most crownward protocetid and identified a strongly supported (PP =  (3)). One reconstructed trait is the posterior margin of the nasal in relation to the posterior margin of the maxilla (PMM) (Character 6; Supplementary Figure 16 and Supplementary Table 9). According to the ASRs, the retention of the nasal extension posterior to the PMM, which is evident in most basilosaurids such as Saghacetus, Pontogeneus, Dorudon, and Basilosaurus, represents the ancestral state of probably all non-basilosaurid archaeocetes. As a result, possessing a posterior nasal edge at or anterior to the PMM in the basilosaurids Ocucajea, Zygorhiza and Neoceti is a derived state that probably evolved convergently in Tutcetus-clade and in Neoceti (Node #73).
Another reconstructed trait is the nasal process of the frontal (Character 16; Supplementary Figure  16 and Supplementary Table 10). According to ASRs, the absence of the nasal process of the frontal, which is evident in some basilosaurids such as Saghacetus and Ocucajea, represents a derived trait in some Neoceti (Node #79). ASRs for the accessory denticles on P3 (Character 106; Supplementary Figure 19 and Supplementary Table 13) reveal that more denticles on the distal edge of the tooth is the ancestral trait of all basilosaurids and Neoceti and is probably retained by their common ancestor (Node #72). Therefore, the equal number of denticles on both edges of the tooth evolved by most basilosaurids such as Zygorhiza, Saghacetus, Pontogeneus, Dorudon, and Basilosaurus is a derived trait that arose again in the early Oligocene mysticete Coronodon. ASRs for the prominent denticulate cingula on P 2-4 (Character 119; Supplementary Figure 20 and Supplementary Table 16) reveals that the absence of the prominent denticulate cingula on P 2-4 is likely the ancestral state of all non-basilosaurid archaeocetes. Therefore, the evolution of this trait represents a derived state that arose in the common ancestor of the Tutcetus-clade (Node #86) and in the basilosaurids Zygorhiza and Pachycetus paulsonii. This trait arose again in the Oligocene odontocete Mirocetus.

Supplementary Tables
Supplementary Table 1 Table 4. Tutcetus rayanensis dental eruption and wear status compared to dorudontine whales. Specimens are listed in the first column in order of age at death from youngest to oldest as determined by their dental eruption sequence. Teeth are listed across the first row in order that they erupt as determined by many dorudontine individuals. Each cell indicates the status of each tooth in each individual using the following code: 0, not present; 1, forming in the crypt; 2, less than half of the crown erupted; 3, greater than half the crown erupted; 4, erupted with no wear; 5, erupted with wear that has not broken the enamel surface; 6, moderate wear with patches of dentine exposed; 7, heavy wear with dentine swaths exposed; 8, very heavy wear with greater than fifty percent of the enamel worn away. A dash indicates that the tooth is not preserved in the specimen. A blank cell indicates that no replacement occurs. The form of the